Grassed waterways (GWWs) are shaped or graded channels that are established with suitable vegetation to convey surface water at a non-erosive velocity using a broad and shallow cross section to a stable outlet.
GWWs efficiently convey runoff from terraces and diversions without causing erosion or flooding. They also prevent gully formation and improve water quality of downstream water bodies by reducing sediment transported by runoff.
The vegetation within GWWs improves soil aeration and enhances water quality due to nutrient (phosphorus, nitrogen) and herbicide removal through plant uptake and sorption by the soil. GWWs also decrease flow velocity, thereby minimizing erosion and they provide a productive wildlife habitat.
How Does This Practice Work?
Due to the establishment of permanent vegetation and the deposition of sediment, phosphorus sorbed onto the sediment remains within the field rather than being deposited in downslope water bodies. Vegetation within GWWs recycles entrapped nutrients in the harvested material and provides a permanent habitat for many types of fauna including native bees.
Where This Practice Applies and Its Limitations
GWWs can be applied anywhere that sufficiently sized land areas contribute contaminants within runoff to a water body and necessitate a reduction in flow velocity. They can be established within agricultural areas where both point and nonpoint source pollution occur, particularly on sites with excessive erosion, leaching and runoff potential.
Advantages of this conservation practices include flood damage prevention; erosion control; aesthetic value; water quality improvement; design based on landowner’s or farmer’s experience; and dispersion of concentrated flow, thereby preventing gully erosion. Lastly, farm machinery can generally cross GWWs.
Limitations of these practices include installation costs (e.g., grading slopes and vegetation establishment), loss of acreage for pasture or crops, and the variability of effectiveness due to the uncertainty of runoff rate and frequency.
Disadvantages of GWWs include the need to implement conservation practices to prevent preferential flow paths, vegetative growth may be troublesome, and the depth of the waterway limits it use as a tile drainage outlet. The construction of the waterway will depend on the soil’s erosive potential. For example, a shallower waterway will result in an area with more erodible soil. The vegetation within GWWs should not be overgrown, since tall growth could trap snow, thereby blocking runoff. Vegetation should be flexible, thereby not reducing flow velocity and its erosive potential.
The effectiveness of GWWs depend upon soil characteristics, land slope/topography impacting drainage into GWWs, the vegetation selected for establishment, and proper construction and maintenance. Wider GWWs will be more effective at trapping sediment and reducing pollutants, due to a larger surface contact area and greater contact time with runoff.
The shape of the waterway greatly impacts the flow velocity and its erosive force, so proper construction and maintenance must occur for the channel to be effective. The waterway should be constructed when there is sufficient time to establish adequate grass growth before the season of high runoff occurs.
The vegetation selected will also impact the effectiveness of GWWs. The vegetation should provide a suitable cover and should be able to establish quickly and form a deep-rooted sod. Seeding should occur perpendicular to the flow of water to further reduce runoff velocity. The waterway should be assessed after large runoff events and bare or eroded spots should be repaired or reseeded.
Cost of Implementing the Practice
Costs are site-specific and depend on the slope, topography, drainage area and length of the waterway. Estimates for engineered GWWs are $1700 per acre, seeding is approximately $30-$40 per acre, and annual maintenance expenses (primarily mowing) are around $20 per acre (Fiener and Auerswald, 2017).
GWWs may be eligible for cost-sharing through programs such as the Conservation Reserve Program (CRP) or Environmental Quality Incentives Program (EQIP), and receive both technical and financial assistance from federal, state, and local levels. Potential returns include revenue from harvesting and marketing grassed waterway hay.
Some additional factors to consider before installing GWWs include:
- types and concentrations of pollutants which may be present in runoff
- soil characteristics, such as clay content, organic material, and infiltration rate
- size of contributing area
- previous or existing vegetation
- steepness of slope/irregularity of topography
- dimensions of the watershed that will be draining into the GWWs
- types of vegetation adaptable to the area
- climatic conditions at planting times
- possible combinations of conservation practices to reduce erosion and chemical loss
- dominant wind direction
Operation and Maintenance
A maintenance program should be established to preserve waterway capacity, vegetative cover, and outlet stability. Vegetation damaged by machinery or herbicides, or erosion must be reestablished promptly.
The operation and maintenance of this conservation practice is minimal once the vegetation is established. The vegetation must receive sufficient moisture and nutrients. However, the waterway should not be so wet as to impede vegetative growth. A saturated waterway will also inhibit accessibility by farm machinery. Drainage tiles may need to be installed to remove water at some locations.
Maintenance for GWWs includes harvesting and marketing forage, repairing rills and removing accumulated sediment. GWWs should be mowed regularly to encourage dense sod establishment. GWWs are considered effective at natural field grade, however, 1-5% slopes have proven to be the most acceptable grade. The minimum expected life for GWWs is 10 years.
Franti, T.G. May 1997. Vegetative Filter Strips for Agriculture. Nebraska Cooperative Extension NF 97-352.
Fiener, P., & Auerswald, K. (2017). Grassed waterways. Precision Conservation: Geospatial Techniques for Agricultural and Natural Resources Conservation, 59, 131-150.
Pfost, D.L. and L. Caldwell. 1993. Maintaining Grassed Waterways. University of Missouri Extension. Report No. G1504.
University of Illinois Extension. July 2003. Plant Vegetative Filter Strips or Make Critical Area Plantings. 60 Ways.
USDA-ARS (1987). Stability Design of Grass-Lined Open Channels. Agriculture Handbook Number 667. Washington, D.C.
USDA-ARS (2020). Conservation Practice Standard. Grassed Waterway. Code 412. Washington, D.C.
USDA-NRCS (Feb 2021). National Engineering Handbook (Title 210), Part 650, Chapter 7, Grassed Waterways. Washington, D.C.
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center. Cost share may be available through your local USDA or conservation district office.
Editing and Design
NC State University
University of Tennessee
Wacha, K. and J. Gilley. 2023. Grassed Waterways. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/grassed-waterways/